"Valve just couldn't wait until GDC 2010 to make "Portal 2" official, and now their other expected announcement for the conference has been confirmed as well. The company wants to port Steam and many of its games over to Mac.

Teaser images spoofing past Apple ad campaigns have been hinting at this revelation for the last week and Mac elements identified in the code for the latest version of Steam all but verified the move. Nevertheless, Valve co-founder Gabe Newell should have a lot of pressure taken off his shoulders now when he gets up to accept his GDC Pioneer Award next week.

Details about how games may or may not be restricted to and distributed outside of the Steam network will likely have to wait until GDC, but Mac enthusiasts should have a good time listening to them when they do, as this is one of the biggest gaming developments to hit their computers in years."

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This sure is a great thing for the Mac gamers.
It's a great move. Also, the games aren't just running through compatibility programs but with the actual code.
And yeah, when you've got the windows version you automatically have the mac version.
I don't think there is an exact release date for it though.

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